Its generally swimmable year round, though you do so at your own risk, as in any lake. If it is definitely not safe to swim (e.g. when and IF blue-green algae makes an appearance) there are signs put up that state as much, e.g., "It is unsafe to swim in this lake at this time due to...." Etc.
Yes you can. When the temperature is up, it tends to become green. And thus not swimming is better.
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